Conventional ultrasound scanners create two-dimensional B-mode images of tissue in which brightness of a pixel is based on intensity of the echo return. Conventional B-mode images are formed from a combination of fundamental and harmonic signal components, the former being direct echoes of the transmitted pulse and the latter being generated in a nonlinear medium, such as tissue, from finite-amplitude ultrasound propagation. In some instances, e.g., obese patients, ultrasound images can be improved by suppressing the fundamental and emphasizing the harmonic signal components.
Propagation of ultrasound beams in biological tissues is known to be nonlinear, giving rise to generation of harmonics. In harmonic imaging, energy is transmitted at a fundamental frequency .function..sub.0 and an image is formed with energy at the second harmonic 2 .function..sub.0. Some of the characteristics of the nonlinearly generated second harmonic beams are: a narrower beam, lower sidelobes than the fundamental, and beam formation in a cumulative process, i.e., the second harmonic continually draws energy from the fundamental during propagation. These characteristics contribute to lateral resolution improvements, reduction of multiple reflections or other aberrations due to difficult windows, (i.e., body locations at which placement of a probe does not result in a good image) and clutter reduction due to inhomogeneities in the tissue and skin layers.
At least two methods for harmonic imaging in an ultrasound scanner are known. In one method, the transducer elements of a phased array are activated by waveforms that have a fundamental frequency and are time-delayed to produce an ultrasound beam which is focused at a transmit focal zone, transmission of a single focused beam being referred to as a "firing". The echoes returned from the body being interrogated are transduced by the array elements into electrical signals and time-delayed to form a receive vector of acoustic data having both fundamental and harmonic signal components. A receive filter removes the fundamental signal component and isolates the harmonic signal component which is then detected, scan-converted and displayed.
In a second method, each transducer element is activated by a first waveform having one polarity during a first transmit firing and by a second waveform having the opposite polarity during a second transmit firing. Both waveforms are broadband pulses having a fundamental frequency. Activations of the transducer elements during each firing are time-delayed to produce an ultrasound beam which is focused at the same transmit focal zone. Each firing results in a respective receive vector of acoustic data, each vector having both fundamental and even harmonic signal components. When the receive vectors are vector summed, however, the fundamental signal components substantially cancel, thereby isolating an even harmonic signal component that is then detected, scan-converted and displayed.
Drawbacks to the first method include the following: (a) the received signal is narrowband and hence resolution is poor; (b) it is difficult to filter the large fundamental signal component completely, so there is some residual fundamental signal that degrades contrast improvement; and (c) if the transmit signal contains harmonic frequencies, it is not possible to filter out those harmonic frequencies.
The second method does not present the disadvantages of the first method. However, a major drawback of the second method is that it requires two firings to acquire harmonic data corresponding to a particular transmit focal zone and hence always decreases the frame rate by half. The second method is also susceptible to motion artifacts. For lower-frequency transducers, the second method is often not realizable.